High carbon chromium steel (JIS G4805: SUJ2) has been widely used as such steel for bearings as described above. Bearing steel is generally required to exhibit excellent rolling contact fatigue life characteristics as one of the important characteristics thereof. The rolling contact fatigue life is presumably shortened by the presence of nonmetallic inclusions or eutectic carbides in bearing steel.
Recent studies have revealed that presence of nonmetallic inclusion in bearing steel is presumably the largest factor of causing deterioration of rolling contact fatigue life characteristics of the bearing steel. Content and size of nonmetallic inclusion have been therefore controlled by decreasing oxygen content in steel to prolong the product life of a bearing.
For example, JP 1-306542 A (PTL 1) and JP 3-126839 A (PTL 2) each propose a technique of controlling the composition, configuration or distribution of oxide-based nonmetallic inclusions in steel. However, PTL 1 and PTL 2 have problems in that the techniques thereof necessitate either introduction of expensive steelmaking facilities or significant modification of the existing facilities in order to manufacture bearing steel with fewer nonmetallic inclusions, resulting in an enormous economic burden on the practitioners.
Further, JP 7-127643 A (PTL 3) discloses a technique of improving the rolling contact fatigue life characteristics of bearing steel by controlling the degree of central segregation of carbon and the contents of oxygen and sulfur in the bearing steel. As mentioned above, however, to further reduce the oxygen content in steel to manufacture bearing steel having an even smaller amount of nonmetallic inclusions, it is necessary to either introduce expensive steelmaking facilities or significantly change the existing steelmaking facilities, leading to a problem with an increased economic burden on the practitioners.
In view of the circumstances described above, attention is now being paid to reducing eutectic carbide in steel, as well as reducing nonmetallic inclusion in the steel. For example, high carbon chromium steel containing carbon by 0.95 mass % or more is very hard and has good wear resistance. However, such high carbon chromium steel has a high degree of segregation at the central portion in cross section of a cast steel product (hereinafter, simply referred to as “central segregation”) and further forms massive eutectic carbides in the casting steel, leading to the problem of poor rolling contact fatigue life. Accordingly, the central portion in cross section of the cast steel product is punched out as a waste material, or alternatively, high carbon chromium steel is subjected to a diffusion process for a long time (hereinafter, simply referred to as “soaking”) to sufficiently dissipate any segregated elements and eutectic carbides from the steel.
To address the problem of segregation, JP 3007834 B (PTL 4) discloses a method for preparing a linear or bar-shaped rolled material having a specific chemical composition containing, for example, C: 0.6 mass % to 1.2 mass %, such that a total area of carbides having a thickness of 2 μm or more, observed in a central region which includes the central axis in a vertical cross section of the rolled material and extends from the central axis by D/8 (D: width of the vertical cross section) on respective sides from a central line in the vertical cross section passing through the central axis of the rolled material, is suppressed to 0.3% or less with respect to the area of the vertical cross section. Further, PTL 4 reveals how the content of massive carbides quantitatively affects the rolling contact fatigue life characteristics, and shows that massive eutectic carbides remain in steel and deteriorate the rolling contact fatigue life characteristics thereof.
JP 5-271866 A (PTL 5) discloses bearing steel that has a specific chemical composition containing particular elements such as C: 0.50 mass % to 1.50 mass % and Sb: 0.0010 mass % to 0.0150 mass %, and that is excellent in heat treatability and productivity with minimal formation of decarburized layers. The technique disclosed in PTL 5 involves adding Sb to the bearing steel for the purposes of reducing the formation of decarburized layers in the bearing steel and improving the heat treatability and productivity of the steel by omitting the cutting or grinding process after the heat treatment of the bearing steel. However, Sb is suspected to be quite harmful to human body and thus requires careful handling when applied to the steel. Further, when Sb is added to the steel, Sb concentrates in the central segregation zone of the steel, exacerbating the central segregation. A portion where Sb has concentrated may cause local hardening, providing a difference in hardness between the portion and the base material while serving as the origin of rolling contact fatigue fracture, which leads to deterioration in the rolling contact fatigue life characteristics of the steel.
In order to dissipate central segregation and massive eutectic carbides in the central segregation zone generated during casting of high carbon chromium bearing steel, JP 3-075312 A (PTL 6) discloses a method comprising rolling cast steel to a billet and subjecting the billet to soaking.
However, the steel has an uneven temperature distribution during soaking and thus the method of PTL 6 has a problem in that the soaking temperature may locally exceed a temperature corresponding to the solidus curve, which triggers local re-melting, causing eutectic reaction to form additional massive eutectic carbides in the steel.
In view of this, low carbon alloy steel can be employed instead of the aforementioned high carbon chromium steel, depending on the use application of bearing. For example, case hardening steel is used as the second most common option after high carbon chromium steel. However, in the case hardening steel, carbon is contained by 0.23 mass % or less, appropriate amounts of Mn, Cr, Mo, Ni, and the like are added thereto to obtain the quench hardenability and mechanical strength required, and surface hardening is performed through carburizing and carbonitriding for the purpose of improving the fatigue strength of the steel.
For example, JP 4066903 B (PTL 7) discloses case hardening steel that can be obtained through carburizing treatment in a short time by specifying the chemical composition thereof containing particular elements such as C: 0.10% to 0.35% and setting the value of activation energy Q for carbon diffusion in the steel, which is defined by the formula: Q=34140−605 [% Si]+183 [% Mn]+136 [% Cr]+122 [% Mo], to be 34000 kcal or less.
Similarly, JP 4050829 B (PTL 8) discloses a technique regarding a carburized material having excellent rolling contact fatigue characteristics and having a specific chemical composition containing particular elements such as C: 0.1% to 0.45%, a carburized layer having an austenite grain size of No. 7 or higher, a carbon content at the surface of 0.9% to 1.5%, and a retained austenite content at the surface of 25% to 40%.
While the aforementioned carburizing and carbonitriding processes improve the rolling contact fatigue life characteristics of the steel, these processes significantly increase the manufacturing cost and decrease the yield due to large strain and dimensional changes, leading to a problem with increased cost of the final products.
In addition, bearing steel is required to have a large cross section depending on the use application thereof, in which case significant modification of the carburizing or carbonitriding facility is required correspondingly, leading to a problem with an enormous economic burden on the practitioners.